Multilayer sheets formed by laminating a synthetic resin onto substrate materials, such as paper and cloth (Herein, these substrate (materials) including paper and materials having like properties are inclusively referred to as “paper-like substrates” or “plant substrate sheets”), are used for various purposes.
For example, paper-made containers, such as paper cups and paper trays, used for food and beverages have been formed by laminating a polyolefin composition as a water-repellent or an oil-repellent layer onto at least one side of a paper-like substrate containing contents, such as liquids or oily food.
In conventional processes for producing materials for paper-made containers, such as paper cups or paper trays, the lamination has to be performed at high temperatures of 300° C. or higher in order to ensure close contact and adhesion of the polyolefin composition as a water-repellent or an oil-repellent layer with paper. Accordingly, the polyolefin is degraded by oxidation to result in residual odor due to the oxidation degradation of the resin composition in the paper laminate, and generation of smoke in a large quantity in the lamination step, leading to problems, such as deterioration of the operation environment and pollution of the surrounding environment.
The paper cups, paper trays, etc., after the use cannot be decomposed even embedded in the earth to pollute the environment because of the lamination of polyolefin composition lacking degradability with microorganisms or hydrolyzability. Accordingly, a composition for a water-repellent layer or an oil-repellent layer capable of biological degradation along with paper has been intensely demanded.
For complying with such demands, various proposals have been made regarding food containers which comprise a multilayer sheet obtained by forming a various biodegradable resin layer on a paper-like substrate and result in little load to the environment at the time of disposal thereof (e.g., Patent documents 1-4 shown below).
On the other hand, food containers are required to show a function of preventing degradation of food contents during storage thereof due to permeation of oxygen or moisture. For imparting the property to paper-based food containers, it has been proposed to dispose a barrier layer of special polymethallyl alcohol on a paper-like substrate (Patent document 5 shown below), but in this case, the product is not likely to be a biodegradable container.
Patent document 1: JP-A 4-336246,
Patent document 2: JP-A 6-171050,
Patent document 3: JP-A 6-316042,
Patent document 4: JP-A 2003-261129,
Patent document 5: JP-A 11-91016,